The only time I can remember using a look-ahead in work code was when I was writing a script that would peer through our CVS tree every Tuesday and print out all of the source code that was modified in the last week. (This forced us to do code reviews on Tuesday because we would walk in to the office and see the heaping pile of code.)
       At any rate, we only wanted to print out certain files in our CVS (we keep everything, there). We did not want to print .txt or .html or other files like that. So, to accomplish this, I did the following:

opendir PROJ, "./$currentProject" or die "Could not open CVS dir $curr
+entProject: $!\n";

my @files = grep /((\w|_)+(?=\.(pl|php|inc|js)$))/, readdir PROJ;

closedir PROJ;
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      I look forward to your book.

Jeremy